Thyroid hormones T3 and T4 are synthesized in a series of steps involving concentrative iodide transport, thyroglobulin synthesis, and iodination of thyroglobulin. The last step is carried out by an enzyme, thyroid peroxidase. A person deficient in thyroid peroxidase would not live normally, having problems such as severe mental retardation. Also, patients with autoimmune thyroid disorders such as Graves' disease and Hashimoto's thyroiditis, produce autoantibodies against thyroid peroxidase by recognizing it as an autoantigen, resulting in destruction of normal thyroid function. Our work has been focused on understanding the molecular mechanism of thyroid hormone synthesis and of thyroid autoimmunity by using various molecular biological approaches. We have identified a thyroid-specific enhancer element of 230 bp that is located approximately 5.5 kbp upstream of the transcription start site of the human thyroid peroxidase gene. Further, we have recently cloned a cDNA encoding a thyroid-specific enhancer-binding protein designated T/EBP that is largely, if not completely, responsible for thyroid specific enhancer activity through its binding to the specific cis-acting enhancer element. A T/EBP protein was expressed in E. coli-based cDNA expression system and was purified to a new electrophoretic homogeneity. The expressed T/EBP was shown to specifically bind to the enhancer-derived double-stranded oligonucleotide by gel mobility shift assays. Various deletion constructs of the upstream DNA sequences of the human thyroid peroxidase gene, connected to a luciferase reporter gene, were examined in detail. Several constructs showed an increased luciferase activity when co-transfected with a T/EBP cDNA-expression vector into nonpermissive human hepatoma Hep G2 cells, suggesting that T/EBP may bind to a sequence(s) other than the previously identified enhancer element.